Field of the Invention
This invention pertains to an external defibrillator adapted to provide therapy selectively to patients suffering from sudden acute cardiac arrest. More particularly, the present invention pertains to an external defibrillator which is constructed and arranged to operate substantially automatically once it is positioned on the patient. The defibrillator rapidly determines the status of the patient, makes a decision on whether therapy is indicated, and, if necessary, applies such therapy until either its operation is discontinued externally or sinus rhythm has been achieved.
The term Sudden Cardiac Arrest or SCA in a patient refers to a condition characterized by a loss of effective pumping action in the heart and is generally caused by an arrhythmia. SCA results in an abrupt cessation of blood circulation to the vital organs, and once it occurs, unless the patient""s heart is reverted rapidly to a sinus rhythm, death will occur. In fact SCA is considered to be the leading cause of death in the United States and throughout the world.
Arrhythmias which cause SCA include ventricular tachycardia and ventricular fibrillation. Ventricular tachycardia is characterized by electrical disturbances which cause a dangerously high cardiac rate and may lead to ventricular fibrillation. Ventricular fibrillation refers to a state where cardiac electrical activity is completely disorganized and the heart is quivering. During ventricular fibrillation, the heart does not pump blood, and no beats can be detected.
Arrhythmias may be detected from the patient""s electrocardiogram (ECG), blood pressure, blood oxygenation level and other similar physiological parameters. Because the signals indicative of these parameters can be very complex, various algorithms are used to analyze these parameters to detect and classify an arrhythmia. Once detected, the arrhythmia can be eliminated by using antitachycardia therapy consisting of electrical stimulation. Two kinds of devices are presently available to provide antitachyarrhythmia therapy: internal or implanted cardioverter defibrillators (ICDs), and external defibrillators.
ICDs have been known since the early 1980s. These devices are implanted in the patient and include electrodes extending to the cardiac chambers to sense intrinsic cardiac activity and to provide stimulation signals. The intrinsic signals sensed in the cardiac chambers are used to classify the condition of the heart, and if a tachyarrhythmia is detected, then either cardioversion pacing pulses or defibrillation shocks are applied.
In order for these kinds of devices to function properly, a clinician examines the patient and, after implantation, programs a plurality of parameters into the ICD which are used by a processor to classify the condition of the patient and determine the characteristics of the stimulation signals to be applied. Frequently these parameters are selelected after the patient""s heart rate is increased either naturally, with exercise, or with drugs. It is advisable to re-program these parameters as the condition of the patient changes over time.
External defibrillators capable of providing defibrillation shocks or other types of therapy are also well known. Current external defibrillators must be operated manually by a trained professional such as an emergency medical technician, paramedic, firefighter, or police officer, etc. Existing external defibrillators do not monitor cardiac activity before a sudden cardiac arrest episode, and accordingly, the professional must examine the patient and determine his condition first, before any therapy is provided. Hence, inherently, the existing external defibrillators cannot be used by a layperson.
An external defibrillator described in commonly assigned U.S. Pat. No. 5,474,574 and incorporated herein by reference includes an ECG sensor and requires several parameters to be programmed by a clinician before it can be used properly. Some of the programmable parameters pertain to the sensitivity of the ECG sensor required to detect ECGs reliably. Other parameters pertain to the size, number and duration of the shocks to be applied by the device. Since these parameters must be programmed separately for each patient, by the time this defibrillator is ready to be used, it is configured to a specific patient and cannot be used for a different patient without first reprogramming its parameters.
In summary, existing external defibrillators are limited in that they must be operated by a professional, they do not have the capability to continuously monitor a patient; and they require active intervention to initiate any therapy.
There is a need for an automatic external defibrillator which can be used successfully by a layman, i.e., a person without any formal medical training.
In view of the above, an objective of the present invention is to provide an external defibrillator which can be distributed and placed at public places which can be used effectively by a person with no special medical training.
A further objective is to provide an external defibrillator able to monitor a patient and determine automatically if a patient is in need of therapy.
A further objective is to provide an external defibrillator capable of providing cardiac therapy without requiring any information about the patient receiving it.
Yet another objective is to provide an external defibrillator which has several modes of operation so that it can be used for different purposes.
Other objectives and advantages of the invention will become apparent from the following description.
Briefly, an external defibrillator constructed in accordance with this invention includes a sensing circuit used to sense physiological signals indicative of cardiac activity, a therapy delivery circuit that generates pacing or shock pulses, and a controller that is used to operate the defibrillator automatically. Signals indicative of intrinsic cardiac activity, including R-waves and ventricular fibrillation, for example, are determined using generic criteria rather than patient-specific programming parameters. Similarly, the pulses applied to the patient to effect therapy have characteristics which are derived from general statistical data and are not patient specific. The defibrillator recognizes a life threatening cardiac condition and can apply appropriate therapy without any human input or intervention.
Optionally, the external defibrillator may include a memory for logging data for each episode during which therapy is applied. A display may also be provided to show instructions for the operation of the defibrillator and/or to selectively display the logged data. A communication module may also be provided to contact remote locations and obtain assistance for the patient.